A study was conducted to determine the relationship
between midday measurements of vine water status
and daily water use of grapevines measured with a weighing
lysimeter. Water applications to the vines were terminated
on August 24th for 9 days and again on September 14th for
22 days. Daily water use of the vines in the lysimeter
(ETLYS) was approximately 40 L vine-1 (5.3 mm) prior to
turning the pump off, and it decreased to 22.3 L vine-1 by
September 2nd. Pre-dawn leaf water potential (WPD) and
midday Wl on August 24th were -0.075 and -0.76 MPa,
respectively, with midday Wl decreasing to -1.28 MPa on
September 2nd. Leaf gs decreased from *500 to
*200 mmol m-2 s
-1 during the two dry-down periods.
Midday measurements of gs and Wl were significantly correlated
with one another (r = 0.96) and both with ETLYS/
ETo (r = *0.9). The decreases in Wl, gs, and ETLYS/ETo in
this study were also a linear function of the decrease in
volumetric soil water content. The results indicate that even
modest water stress can greatly reduce grapevine water use
and that short-term measures of vine water status taken at
midday are a reflection of daily grapevine water use.